feat(block2): offline outbox — share now, publish when connected

Sender-side offline delivery hardening.
- OfferOutbox: a keystore-backed (no plaintext) queue of lot ids to publish
  later. Stores ids, not offers, so on flush each offer is rebuilt from the
  lot's CURRENT state — an offline edit is respected, a deleted lot drops out.
- flushOutbox(): on reconnect, republishes queued lots and clears them (no-op
  offline; drops ids whose lot is gone).
- Market screen: sharing offline parks the lots ('we'll share these when you're
  connected'); opening the market online auto-flushes the queue first.
- Wired via DI + TaneApp(outbox); i18n en/es/pt.

Tests: 4 outbox + 3 flush + market UI, 20 green. Analyzer clean for new code.

apps/app_seeds/lib/main.dart

@@ -6,6 +6,7 @@ import 'data/variety_repository.dart';
 import 'di/injector.dart';
 import 'i18n/strings.g.dart';
 import 'services/coarse_location.dart';
+import 'services/offer_outbox.dart';
 import 'services/onboarding_store.dart';
 import 'services/social_service.dart';
 import 'services/social_settings.dart';
@@ -24,6 +25,7 @@ Future<void> main() async {
         social: getIt<SocialService>(),
         socialSettings: getIt<SocialSettings>(),
         location: const GeolocatorCoarseLocation(),
+        outbox: getIt<OfferOutbox>(),
         showIntro: !await onboarding.introSeen(),
       ),
     ),